1. Field of the Invention:
The invention relates to a process of fractionally desublimating solids in vapor form.
2. Description of the Background:
The invention has the purpose of making the desublimation of solids in vapor form from gas/vapor mixtures by means of cooling gases simpler and less prone to faults.
Desublimation is the physical process in which there is a direct transition of a solid in the vapor state form to the solid form.
A number of inorganic and organic solids are prepared by initially forming the solid as a vapor in a carrier gas and then fractionally desubliming by cooling of the gas/vapor mixture. This fractional desublimation can be accomplished by cooling the gas/vapor mixture on temperature-controlled wall surfaces, the equipment frequently being provided with mechanical scraping devices. Another possibility of desubliming solids from the gas phase involves direct cooling of the gas/vapor mixture by means of coolants which are gaseous under the desublimation conditions. In the process of British Pat. No. 1,081,579, for example, water, as a preferred coolant, is injected under pressure into the gas/vapor mixture and vaporized. In this process, however, a part of the solid by-products is also precipitated, in addition to the solid main product, on the water droplets before the latter have been fully vaporized, so that fractional desublimation is hardly possible. Because of the preferred use of water, the process is also unsuitable in cases where water-sensitive substances such as, for example, acid anhydrides are to be desublimed. Since the walls of the desublimator are heated and the residence time of the gas/vapor coolant mixture in the desublimation zone is very long, a part of the coolant is additionally heated by the walls, so that large quantities of coolant are required. This applies very particularly when cooling gases are used, with which the high latent heat of vaporization of water is not available for cooling. Cooling gas is additionally also introduced at the bottom of the desublimator. As a result, the total quantity of coolant required, and hence also the quantity of exit gas, are further increased.
Another process is described in German Pat. No. 1,108,663. Here, the carrier gas, already freed from product, is circulated through coolers and mixed with the product-carrying gas. At the same time, liquid product is injected into the mixing chamber. Fractional desublimation is hardly feasible by this process.
French Specification No. 2,082,822 shows a method in which desublimation is performed with air in two series-connected pipes, in order to achieve better precipitation of the solid on the pipe walls and to obtain a purer solid. This process requires large wall areas which cannot be realized industrially, and the pipes are rapidly blocked by the precipitated solid.
In a similar process, the cooling of the gas/vapor mixture is carried out by means of cooling gases and with the use of additionally cooled pipes provided with scraping devices, as described in German Patent Specification No. 2,617,595. In this expensive process which is prone to faults, the highly disperse part of the product stream is circulated. Fractional desublimation is again impossible.
Yet another desublimation process is described in German Patent Specification No. 1,544,129, in which the carrier gas containing the solid in vapor form and the cooling gas form two coaxial gas cylinders which rotate in the same direction and move axially in opposite directions, and in which the sublimed solid is discharged from the desublimator with a part of the cooling gas. In this process, a very large quantity of cooling gas must be used in order to prevent the gas/vapor mixture from coming into contact with the wall, and in order to discharge the desublimed solid. In this process, too, fractional desubimation is not possible.
In German Offenlegungsschrift No. 3,501,371.0, the fractional desublimation of solids in vapor form from a gas/vapor mixture is accomplished by means of a cooling gas, in such a way that the gas/vapor mixture, accelerated by means of a nozzle, and the cooling gas, likewise accelerated by means of a nozzle, impinge on each other at very high flow velocities, the nozzles being located opposite one another and the nozzle axes lying on a straight line. In this process, depositions of material on the nozzles or blockages of the nozzles with material can take place, if wide irregularities occur in the feed of the gas/vapor mixture or of the cooling gas. If, for example, the feed of the gas/vapor mixture is briefly interrupted or very greatly reduced, the cold cooling gas can strike the nozzle of the gas/vapor mixture and cool the latter to some extent, so that the solid which is to be desublimed can in part already precipitate on the nozzle orifice and effect a deflection of gas/vapor mixture. Since the two gas streams then no longer impinge frontally on one another, retarded and incomplete mixing of the two gas streams is likely to occur, so that a more impure product is obtained. The same can also happen when, conversely, cooling gas is fed improperly, the result of which is that the gas/vapor mixture strikes the cold cooling gas nozzle and the solid, which is to be desublimed, precipitates partially on the cold cooling gas nozzle. A need therefore continues to exist for an improved technique of fractionally desubliming solids.